1. Field of the Invention.
The present invention relates to a zoom lens system for use in an image projecting apparatus with Kohler illumination, and more particularly to a zoom lens system wherein a pupil position at the reduction side is made substantially invariable in an operation of zooming so as to satisfy the conditions of Kohler illumination without changing the position of a condensing lens system in Kohler illumination.
2. Description of the Prior Art.
Kohler illumination has frequently been used in a projecting optical system since it has conventionally given a high illumination efficiency. One example of such projecting optical system employing Kohler illumination is a system for projecting images of microfilms.
A microfilm image projecting optical system employing conventional Kohler illumination will be explained hereinbelow with reference to FIG. 1. Specifically, the projecting optical system has a zoom lens system comprising, from the enlargement side, a first lens group 5 of a negative refractive power, an aperture stop 7 and a second lens group 6 of a positive refractive power. This zoom lens system projects an image on a film 4 onto a screen 8 with the magnification varied. In order to illuminate the film 4, luminous flux from a light source 1 is callinated by a first condensing lens unit 2. Then, the image of the light source 1 is formed at the position of the aperture stop 7 by a second condensing lens unit 3. In this way, the filament image of the light source 1 is formed at the pupil position of the zoom lens system, with the result that the conditions of Kohler illumination is satisfied as well as that the image having a effective illumination density is projected on the screen 8.
However, the position of the aperture stop 7 is altered in an operation of zooming from the longest focal length side shown in FIG. 1A to the shortest focal length side shown in FIG. 1B. Accordingly, in this projecting optical system, it is necessary that either the first or second condensing lens group 2 or 3, or alternatively the whole condensing lens group is moved to fulfill the conditions of Kohler illumination.
Namely, in order to maintain the conditions of Kohler illumination in the operation of zooming, the condensing lens system has to be manually adjusted by users or to be moved in a mechanically interlocking relationship with the zoom lens system, causing trouble in operating the machine or entailing an increased cost to the machine.